In short, nothing.
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We just have to accept that it's a minority choice and the only way to really promote it is by word of mouth.
I disagree that it's hopeless.
The first step is to admit that W10M currently does suck and deserves to be exactly where it is. Its few redeeming qualities simply can't make up for all the issues it has (quality wise, missing functionality (particularly outside the U.S.), continually sacrificing good ideas to become almost the most generic smartphone OS imaginable, the app gap, etc).
With the exception of the app gap and becoming very generic, most of that can be fixed, and must be before it's worth taking the next steps. Advertising for something like W10M, in its current form, is pretty much equivalent to just throwing money away.
Then, and only then, MS must finally... finally... FINALLY... start thinking bigger!
For example, I'd start thinking about how W10M might work as a companion OS/device for HoloLens. Maybe most of the processing power could be built into a W10M tablet, with the glasses being nothing more than a very light WiDi display, or maybe a W10M device could act as an advanced controller for HoloLens based games? Maybe a W10M tablet could be used to provide a HUD to every car. Maybe W10M and Cortana could be put at the center of every Smart Home, not just as a way to interconnect the low-end junk, but also as a direct competitor to high-end Crestron or KNX based setups. All of the above and more would be my approach. MS just needs to start doing stuff that goes beyond the typical smartphone stuff. MS needs a few niches where W10M can become the dominant player, and then grow from there. Right now there is not a single thing W10M excels at, and nothing it does really breaks out of the smartphone mold. MS had a huge spur of innovation while creating WP7.0. Since then their track record for mobile OS innovation has been pathetic. Fix that and they can have their comeback.
The way the mobile division is currently being run I just don't see much hope for that happening.